Field of the Invention
The present invention relates to a zoom lens and an image pickup apparatus including the same, which are suitable for an imaging optical system such as a digital still camera, a digital video camera, a TV camera, and a monitoring camera.
Description of the Related Art
Hitherto, an imaging optical system used in an image pickup apparatus has been required to have a high optical performance over the entire object distance from infinity to close-up. Moreover, an image pickup apparatus capable of realizing focusing at high speed and with high precision has been required. In particular, when autofocus is performed, a focus speed has been required to be high.
Meanwhile, in recent years, an image pickup apparatus such as a single-lens reflex camera has been required to have a moving picture photographing function and to be able to perform autofocus while shooting a moving picture. As an autofocus method when the moving picture is taken, the contrast AF method (TV-AF method) in which a change in contrast in an imaged signal is detected to evaluate a focus state of the imaging optical system has often been used.
In the TV-AF method, fast small vibration in an optical axis direction is generated in a focus lens unit, and the imaged signal obtained at that time is used. In the TV-AF method, the focus lens unit is required to be small in size and light in weight in order that the focus lens unit may be driven at high speed, and further in order that a load on a drive unit (actuator) for driving the focus lens unit is reduced to maintain silence.
Hitherto, there has been known a zoom lens in which, of a plurality of lens units constituting the zoom lens, a lens unit that is relatively small in size and light in weight is moved for focusing.
In Japanese Patent Application Laid-Open No. 2003-295060, in a zoom lens including first to fourth lens units respectively having positive, negative, positive, and positive refractive powers in order from an object side, the second lens unit, or the first lens unit and the second lens unit is/are moved in an optical axis direction for focusing.
In Japanese Patent Application Laid-Open No. 2009-251114, in a zoom lens including first to fifth lens units respectively having positive, negative, positive, positive, and positive refractive powers in order from an object side to an image side, in which the lens units are moved for zooming, the third lens unit is used for focusing.
In Japanese Patent Application Laid-Open No. 2012-247687, in a zoom lens including first to fourth lens units respectively having negative, positive, negative, and positive refractive powers in order from an object side, the second lens unit is moved in an optical axis direction for focusing.
In order to obtain a zoom lens having the high optical performance over the entire object distance while reducing the size of the entire system, and in addition, being capable of focusing at high speed, it is important to appropriately configure a zoom type, the number of focus lens units and moving conditions thereof, and the like. It is especially important to appropriately set refractive powers of lens units arranged on the object side and the image side of the focus lens unit, a lens configuration of the focus lens unit, and the like.
When the above-mentioned configurations are inappropriate, it becomes difficult to obtain a zoom lens which secures a predetermined zoom ratio and has the high optical performance over the entire object distance from infinity to close-up while reducing the size of the entire system. For example, when a lens unit that mainly plays the role of varying magnification in the zoom lens is used for focusing, a variation in aberration accompanying the focusing is increased, and for the purpose of correcting the variation in aberration, there is a need to increase the number of lenses constituting the lens unit that mainly plays the role of varying magnification. Then, the focus lens unit itself inevitably increases in weight, which makes the focusing at high speed difficult.
Moreover, when a lens unit that is small in size and light in weight is used for focusing, unless a refractive power of the focus lens unit is set appropriately, the variation in aberration is increased during focusing, which makes it difficult to obtain the high optical performance over the entire object distance.